The invention relates generally to antenna sidelobe reduction systems and more particularly to a system utilizing antenna phase center modulation.
The antenna pattern for a directional antenna includes a mainlobe and several sidelobes. The sensitivity of the antenna to these sidelobes decreases the directivity of the antenna. Various methods of reducing sidelobe sensitivity include amplitude tapers across the antenna aperture and coherent sidelobe cancelers. However, local scatterers in the vicinity of the antenna and phase and amplitude errors limit the sidelobe sensitivity reduction achievable by these methods.
An alternative method of sidelobe sensitivity reduction provides for rapidly moving the phase center of the antenna to Doppler shift the rf pulses transmitted into the sidelobes. The Doppler shifted echos from off-axis targets are blocked by a filter thereby decreasing sidelobe sensitivity. U.S. Pat. No. 3,412,405 issued to Crotty et al. discloses this type of system.
In order to induce the Doppler shifts in sidelobe pulses, the rate of change of the phase center position must be extremely high. This high speed phase center modulation requires a costly and complex modulation system.